This invention relates to a method of manufacturing a product based on a simple or mixed metal oxide, or silicon oxide, from one or more organic precursors using supercritical CO2 as a reaction medium.
These metal oxides can be, for example, oxides of titanium, aluminum, magnesium, thorium, barium, beryllium, zirconium etc.
This method, depending on the operating conditions leads to products in the form of liquids, gels, powders, fibers etc being obtained.
One of the objectives of the invention is the formation of a powder, with a particle size and a particle geometry that is controllable as a function of the operating conditions used for its manufacture and the method of separation of the product obtained. Furthermore, the method of the invention permits the manufacture of a powder composed of nanometric particles.
These powders find application, for example, in the manufacture of ceramic materials, this manufacture requiring specially developed raw materials.
In effect, by doping a Si3N4 ceramic with 5% SiC, in the form of nanometric particles, a nanocomposite ceramic is obtained five times more resistant to the propagation of cracks than the initial Si3N4 ceramic. The nanoparticles are used here as dampers, absorbing applied mechanical stresses and allowing atomic planes to slide.
In addition, by using a nanocomposite ceramic formed only from nanometric particles, a nanocomposite is obtained with properties of toughness and ductility comparable to steel. Prospective applications are, for example, for engines, turbines and in space, for example as a refractory coating for space craft.
From the point of view of medical research, nanocomposite ceramics are being studied for the production of prostheses, for example hips and vertebrae, which are mechanically strong and compatible with the human body which is not the case for steel.
Furthermore, in the manufacture of these ceramic materials, an amorphous structure of the initial powder permits, in certain cases, sintering at a lower temperature.
Another example of a use of these compounded powders is the doping of ferromagnetic materials. In effect, when doped by nanoparticles, the ferromagnetic materials can acquire strong magnetization under a very small energizing field. This phenomenon, known by the name xe2x80x9csuper-paramagnetismxe2x80x9d has a direct application in the improvement of reading heads for magnetic tapes and disks.
The method of manufacture according to the invention, also allows the manufacture of metal oxide powders which could find application in optics, for example, as surface coatings to improve the absorbing power in the visible spectrum, while at the same time reducing the losses of infra-red radiation.
Numerous other examples of uses of these powders based on oxides can be mentioned such as the manufacture of micro-porous solids used as catalysts, the manufacture of stationary phases for solid phase chromatography, the manufacture of selective membranes for nanofiltration, the separation of gases etc.
The production of an oxide powder can be provided from four manufacturing routes, the solid, liquid, gaseous and supercritical routes.
The solid routes very often require the application of a mechanical step, for example, grinding, abrasion etc. in order to obtain a desired particle size distribution for the powder. These methods, economically cost-effective, do not allow one to control precisely the size of the final particles and in particular to develop sub-micron particles.
The major problem with production of powders by a liquid route is the agglomeration of particles. In effect, the removal of the solvent, the seat of the reactions, generally brings about the partial agglomeration of the particles, sometimes making it difficult to use them industrially.
The sol-gel method also permits the manufacture of a fine metal oxide powder. This method consists of preparing a stable suspension of condensed species, in a liquid, from precursors (mineral salts or organo-metallic compounds). This suspension forms, from these condensed species, an amorphous three dimensional network in the sol that imprisons a fraction of the liquid, leading to the formation of a gel. The powder is obtained by the total removal of the liquid from this gel.
The precursors used in the sol-gel route for the preparation of oxide powders are organo-metallic precursors such as alkoxides or mineral precursors such as metal salts or hydroxides.
When the precursors are alkoxides, their activity can be modified notably by using complexing agents such as acetyl acetone which blocks the alkoxy groups. One then obtains modified alkoxides.
The gaseous routes do not allow one to obtain amorphous oxide powders because of the high temperatures generally used.
As for the supercritical routes, they are used in various techniques for the preparation of powders, for example, the hydrothermal synthesis technique, supercritical drying and reactions in a supercritical medium.
Hydrothermal synthesis is carried out under supercritical conditions, that is to say at a pressure greater than 2.2xc3x97107 Pa, and at a temperature greater than 374xc2x0 C. Water is used for the development of large crystals by slow crystal growth.
Supercritical drying consists of removing a solvent while circumventing its critical point, that is to say by passing in a continuous way from the liquid to the gaseous state.
As for reactions in a supercritical medium, the document The Journal of Supercritical Fluids 4, p.55, 1991, describes a study of the solubility and the thermal resistance of organo-metallic compounds in supercritical CO2 at 150 and 170xc2x0 C. for a range of pressures between 1.2xc3x97107 and 2.2xc3x97107 Pa.
The document The Journal of Material Science 27, 1992, 2 187-2 192, describes the synthesis of sub-micron MgAl2O4 powders in a supercritical ethanol medium from the double alkoxide Mg[Al(O-SecBu)4]2. This synthesis is carried out at about 360xc2x0 C.
The document Materials Chemistry and Physics 32, 1992, pages 249 to 254, describes the synthesis of sub-micron powders of titanium oxides, in the vapor, liquid and supercritical phase. The synthesis of titanium oxide in supercritical phase is carried out at about 350xc2x0 C. in a supercritical ethanol medium.
The document Silicates Industriels, 1994, 3-4, pages 141 to 143, describes the use of supercritical fluids as reaction media for the synthesis of ceramic powders. The powders formed are powders of titanium oxides and of the spinel MgAl2O4. The supercritical fluid used is a supercritical ethanol medium and the reaction temperature is about 360xc2x0 C.
The invention relates to a method of manufacturing a product based on a simple or mixed metal oxide, or silicon oxide, from a charge of one or more precursors comprising one or more organo-metallic precursors, said method comprising bringing the charge of precursor(s) into contact with a reaction medium comprising supercritical CO2, at a temperature of from 31 to 50xc2x0 C. and a supercritical pressure of from 107 to 5xc3x97107 Pa in order to form, from said precursor, a product based on a simple or mixed metal oxide or silicon oxide; and the separation of said product based on a simple or mixed metal oxide or silicon oxide, or organo-metallic product(s), from the reaction medium by reducing the pressure of the supercritical CO2 to a pressure lower than the supercritical pressure.
The precursor charge may comprise one or more organo-metallic precursors only, but may also include, in addition to the organo-metallic precursor(s), organic compounds, such as, for example, iso-propanol, acetyl acetone etc.
The precursor or precursors are, for example, alkoxide precursors, identical to those used for the sol-gel route.
These precursors can be modified, notably by complexing agents such as acetyl acetone which blocks the alkoxy groups and thereby reduces the reactivity of the alkoxide with respect to hydrolysis.
These alkoxide precursors are, for example, tetra-ethoxy silane, titanium (IV) iso-propoxide, aluminum iso-propoxide, magnesium ethoxide or a mixture of these alkoxides.
When the alkoxide precursor is tetra-ethoxy silane, it may, for example, be used alone as the precursor charge, that is to say without a solvent.
When the alkoxide precursor is titanium (IV) iso-propoxide, it may, for example, be used alone as the precursor charge, that is to say without a solvent
When the alkoxide precursor is a mixture of aluminum iso-propoxide and magnesium ethoxide, the precursor charge can, for example, be formed from a mixture of first and second individual solutions; the first individual solution comprising, for example, magnesium ethoxide, iso-propanol and acetyl acetone, and the second individual solution comprising, for example, aluminum iso-propoxide, iso-propanol and acetyl acetone.
According to the method of the invention, the reaction medium may contain only supercritical CO2, or supercritical CO2 in the presence of a co-solvent, for example, water or ethanol. The co-solvent is preferably in a minority concentration in the reaction medium.
When the co-solvent is water, the method consists of a hydrolysis-condensation of the alkoxide in a supercritical CO2 medium. The water enables one, for example, to provide a wide range of textures of the manufactured product in the case of products based on silicon oxide and titanium oxide.
When the co-solvent is, for example, ethanol
the ethanol will allow the preferential dissolution of the organo-metallic precursor(s) in the supercritical CO2 and therefore permit the reaction itself;
the ethanol, as an alcohol, also allows association of organo-metallic precursor molecules in the form of a dimer, or a trimer, making them more or less reactive.
it can exchange with the alkoxide groups of the precursor(s) and modify its reactivity; for example, the exchange between a propoxide group and the ethanol on the metal in order to form an ethoxide increases the reactivity of the precursor.
Other co-solvents can be used for the same purpose, such as aliphatic or aromatic solvents or halogenated solvents.
According to the invention, bringing the charge, comprising said precursor into contact with a reaction medium comprising the supercritical CO2, can be carried out, for example, for a duration ranging up to 72 hours, preferably 20 hours. A long contact time between the charge of precursor(s) comprising one or more organic precursors and the medium comprising the supercritical CO2 permits good initiation of the process of forming the product based on a metal oxide.
The supercritical CO2 is used as a solvent and as a reaction medium, it has the physical and chemical properties of a liquid and of a gas. Supercritical CO2 encourages the collision of the molecules in the reaction medium and because of this improves the reaction kinetics between these molecules.
According to the invention, a reaction medium comprising supercritical CO2 is used at a temperature of from 31 to 100xc2x0 C., preferably from 31 to 50xc2x0 C. and more preferably 40xc2x0 C.
The supercritical pressure can be, for example from 107 to 5xc3x97107 Pa, preferably 3xc3x97107 Pa.
This method is therefore completely different to that described in the document Silicates Industriels, 1994, 3-4, pages 141 to 143, in which the supercritical medium is a supercritical ethanol medium and the supercritical temperature is of the order of 360xc2x0 C.
The separation of the product based on the oxide formed, from the reaction medium, can be carried out by reducing the pressure of the supercritical CO2, to a pressure lower than the supercritical pressure of the CO2 and at a constant temperature. This pressure reduction of the supercritical CO2 can be carried out by several procedures, for example by a xe2x80x9cone timexe2x80x9d procedure, in order to recover in one operation, all of the oxide based products formed.
The supercritical pressure reduction can also be carried out in several pressure reduction stages that allow one to obtain different fractions of the product formed, which differ from one another by the size and/or the structure of the particles which make up this product.
When the pressure reduction is carried out in stages, the pressure reduction stages each correspond preferably to pressure drops of from 105 to 5xc3x97106 Pa.
The pressure reduction of CO2 can also be carried out in a progressive manner without staging the pressure reduction. This progressive pressure reduction is also called xe2x80x9cslow pressure reductionxe2x80x9d.
Depending on the pressure reduction procedure used (rapid, staged or progressive), one can direct the growth or the formation towards a particulate morphology or a fibrillar morphology. This has been brought to the fore, for example, when using silicon alkoxide.
When the pressure reduction is progressive, it is from 105 to 107 Pa/min., preferably from 105 to 106 Pa/min., and more preferably at 5xc3x97105 Pa/min.
The method according to the invention can be carried out in any system comprising the following elements: a reservoir of CO2, in the liquid state, a high pressure CO2 pump, a hot exchanger used to bring the super-pressurized CO2 to the supercritical state, a sealed pressurizable reaction tank, for example an autoclave, in which the contacting of the precursor charge and the reaction medium comprising supercritical CO2 is carried out, means of reducing the pressure from the supercritical pressure, and means of recovering the products formed, these elements being linked by a sealed and pressurizable circuit.
This type of installation allows one, for example, to recover the products formed directly in the reaction vessel, by reducing the pressure of the supercritical CO2. This may, for example, be the xe2x80x9cone timexe2x80x9d pressure reduction of the SC CO2 described above, for the recovery, in one operation, of all the oxide based products formed in the reaction vessel. The pressure reduction will be carried out, for example, by causing the CO2 to escape through vents.
When a co-solvent is used, for example, water or ethanol, a second vessel can be put in series with the first reaction vessel in the sealed circuit. This second vessel subsequently called the solvent vessel, is sealed and pressurizable, and is, for example a second autoclave that includes the co-solvent. In this case, when a co-solvent is being used, the CO2 is first passed into the second autoclave containing the co-solvent, that is to say, the water or the ethanol, and then it is passed into the reaction vessel. In this case, the product can also be recovered xe2x80x9cin a single operationxe2x80x9d in the reaction vessel, for example, by causing the CO2 to escape through vents.
In order to obtain different fractions of the product formed which differ in the size and/or the structure of the particles that make up the product, from the reaction medium contained in the extractor, a system can be used that allows controlled pressure reductions of the supercritical CO2 to be made followed by successive recoveries of the products. The recovery of the product is carried out here by pressure reduction stages. The system is connected to the reaction vessel by a sealed and pressurizable circuit.
An example of such a system is the separation system described in the patent application EP-A-0 238 527. This system comprises different separators, called xe2x80x9ccyclone separatorsxe2x80x9d and permit one to fractionate the products arising from the contact of the charge of precursor(s) with the reaction medium comprising the supercritical CO2. The cyclone separators operate according to the principle according to which:
a pressure drop of a supercritical solvent causes a decrease in the solvent power of this supercritical solvent. Hence a pressure drop of a supercritical mixture causes a decease in the solubility of the products which are dissolved in it.
In this cyclone separator, the pressure drop is associated with a xe2x80x9ccyclone effectxe2x80x9d, that is to say, the supercritical mixture is injected into the upper part of the cyclone separator (an autoclave with a conical shape), tangentially to the side wall. This type of injection accentuates the segregation between the supercritical solvent and the products which are dissolved in it, the solvent is discharged through the upper part of the cyclone separator, while the product is extracted in the lower part of this cyclone separator.
The mixture can, for example, be subjected to a first pressure reduction in a first cyclone separator enabling the collection of a first fraction comprising the oxide based products that are the least soluble in supercritical CO2, that is to say the heaviest.
The reaction medium resulting from this first pressure reduction can, for example then be passed through a sealed and pressurizable circuit to a second cyclone separator so as to be subjected to a second pressure reduction and permitting the collection of a second fraction comprising oxide based products that are less heavy than those collected in the first fraction.
At the same supercritical pressure as that of the second cyclone separator, the mixture resulting from this second cyclone separator can then, for example, be passed into a recovery column comprising liquid CO2. The oxide based products, which are the most soluble, that is to say the lightest are then recovered in a final fraction.
In order to recover the product by slow pressure reduction of the supercritical CO2, a system analogous described for pressure reduction in stages, can, for example, be used.
According to the invention, the recovery of the product based on a simple or mixed metal oxide, or silicon oxide, can be followed by an aging treatment step for said product for a period ranging, for example, up to 45 days. This aging treatment, subsequent to the supercritical treatment is, preferably, of the order of 30 days.
The aging treatment can be carried out in the supercritical CO2 medium or outside of it.
The aging treatment is, for example, carried out at ambient temperature, for example, about 25xc2x0 C.
It allows the growth of particles from species formed in the SC CO2 in the presence or not of a co-solvent, that is to say, the formation of definitive materials.
When this treatment is carried out in the supercritical CO2 medium, it is carried out at a pressure ranging from 107 to 5xc3x97107 Pa.
According to the method of the invention, when the desired product is a product based on titanium oxide, the organic precursor can be titanium (IV) iso-propoxide. Hence the precursor charge will include titanium (IV) iso-propoxide.
When the desired product is a product based on silicon oxide, the organic precursor can be tetra-ethoxy silane. Hence the precursor charge will include tetra-ethoxy silane.
When the desired product is a product based on magnesium oxide and aluminum oxide, the organic precursors can be magnesium ethoxide and aluminum iso-propoxide. Hence the precursor charge will include magnesium ethoxide and aluminum iso-propoxide.
The method of the invention allows one to develop new types of materials based on oxides chosen from among the oxides of titanium, aluminum, magnesium, thorium, silicon, yttrium, barium, beryllium, zirconium, vanadium, hafnium, scandium, chromium, niobium, molybdenum, lanthanum, tantalum, tungsten etc., using supercritical CO2 as a reaction medium.
The alkoxide precursors used are the same as those traditionally used for the sol-gel route and bringing them into the presence of CO2 in the supercritical state has lead to original products being obtained in the form of a liquid, gel, powder, fiber etc.
For example, for products based on Si oxide, the operations carried out lead to the formation of a wider spectrum of textures than those observed in the prior art. The textures obtained, are, for example, in the form of fibers, small spherical particles with or without points, conical shapes, gangues etc.
The operating conditions, for example the choice of co-solvent, the mode of pressure reduction, the level of sampling and the contact time permits one to direct the texture of the desired final product. In particular, moist Si gels manufactured according to the method of the invention have pore volumes (measured by thermoporometry) of about 0.5 to 1.5 cm3.gxe2x88x921, much greater than those usually measured for Si gels obtained by the traditional sol-gel route from the same precursor. Furthermore, gels dried at 100xc2x0 C. keep high pore volumes of from 0.5 to 0.7 cm3/g and high specific surfaces of from 480 to 760 m2/g.
For products based on Ti oxide, the method of the invention additionally permits the formation of an amorphous powder, dry or in solution which has very interesting textural characteristics in comparison with the powders traditionally obtained through the sol-gel route. In particular, specific surfaces for a powder ranging from about 400 to 500 m2.gxe2x88x921, have been obtained, that is to say twice as high as those obtained by the sol-gel route from the same precursor, for a smaller and relatively homo-dispersed particle size distribution. The pore volume of the titanium oxide powder obtained by the method of the invention is from 0.2 to 0.3 cm3.gxe2x88x921.
From a practical point of view, the spherical morphology of the powders obtained by the method of the invention leads to better reactivity to sintering up to 1100xc2x0 C. This type of powder, pretreated at 450xc2x0 C. before compaction, allows access to materials consolidated at a lower temperature than those derived from the traditional sol-gel method.
For example, for products based on oxides of Mg and Al, the method of the invention has lead to the formation of several crystalline forms of acetyl acetonate.
Taken as a whole, the products formed are of a new type and they allow one to carry out the development of materials with interesting properties.
It is possible to mention, for example, the tangential filtration membranes with a compressed layer of TiO2 and the catalysts.
The characteristics and advantages of the invention will better become apparent on reading the description that follows. This description rests on embodiment examples, given for explanation purposes only, being not limitative and making reference to the appended FIGURE.